Review usages of Coypu.ElementScope.this[] and replace with GetAttribute() if useful

showed that using accessing a nonexistant HTML attribute on an ElementScope should throw an exception in some cases. This lead to the introduction of the ElementScope.GetAttribute() extension method, throwing a MissingHtmlException if an attribute cannot be found.

Review the current usages of Coypu.ElementScope.this[] and check if GetAttribute() might be a better fit.